Is He Dead?

Are the zombies in "The Crazies" real zombies? Maybe, maybe not. Is there an agreed definition of what is a zombie and how they get that way? Not that I know of. I think zombies are defined by behavior and can be "explained" by many handy shortcuts: the supernatural, radiation, a virus, space visitors, secret weapons, a Harvard education and so on. I suppose it would be a "spoiler" if I revealed why the Crazies are on the lurch, but come on, does it matter? What if I revealed they got that way because of, oh, say, eating Pringles? Would that spoil things for you? What difference does it make? All that matters is that they got to be zombies somehow.